As shown in FIG. 8, in many hydraulic auto-tensioners 100, a low-pressure oil chamber 106 is formed between a casing 101 and a cylinder 102 provided on the inside thereof, a high-pressure oil chamber 105 pressed by a plunger 103 is formed in the interior of the cylinder 102, and by biasing a return spring 104 in a direction for causing the plunger 103 to retreat, oil L flows in a single direction from the low-pressure oil chamber 106 to the high-pressure oil chamber 105 via a check valve 107. Meanwhile, during an overload, when the plunger 103 advances deeply into the cylinder 102, the oil L in the high-pressure oil chamber 105 leaks into the low-pressure oil chamber 106 through a clearance G between the cylinder 102 and a piston portion 111 of the plunger 103 from an end of the clearance G near an upper end surface of the cylinder 102.
However, when the up-down stroke of the plunger 103 is large or the frequency is high during the overload, the oil L is ejected forcefully from the end of the clearance G into the air above the oil L in the low-pressure oil chamber 106 through a tapered opening 110 formed at the end of an inner peripheral surface of the cylinder 102, as shown in FIG. 9, and falls into the oil L in the low-pressure oil chamber 106 after mingling with the air. As a result, the amount of air 109 mixed into the oil L in the low-pressure oil chamber 106 gradually increases. Eventually, the air 109 flows into the high-pressure oil chamber 105 through the check valve 107 together with the oil L, and as a result, a backlash corresponding to the volume of the air 109 is generated in the plunger 103, leading to deterioration of the auto-tensioner function, an increase in vibration of the belt or the like serving as the subject of tension application, and abnormal noise. Various devices have been proposed in response to this air problem.
As a device for forestalling air mingling, Japanese Patent Application Publication No. H9-203447 describes a device in which a ring-shaped ejection groove is provided in recessed form in the inner peripheral surface of the upper portion of a cylinder, an ejection hole is formed to penetrate the groove bottom of the ejection groove, and oil L in a high-pressure oil chamber is discharged directly into the oil L in a low-pressure oil chamber from a clearance and through the ejection groove and the ejection hole.
Further, as a device for dealing with the occurrence of air mingling, Japanese Patent Application Publication No. 2001-116092 describes a device in which a baffle is provided in a low-pressure oil chamber to form a narrow gap, through which oil can pass but air bubbles are unlikely to pass, between itself and a casing. Thus, air bubbles mixed into the oil in the low-pressure oil chamber are prevented from entering a high-pressure oil chamber.